There is at present growing interest in non-radioactively labelled modified oligodeoxynucleotides. Biotin (Agrawal. S. et al., Nucleic Acid Research, 14:6227-6245 (1986); Agrawal, S. Tet. Lett., 30:7025-7028 (1989)), florophores (Cardullo, R. A. et al., Proc. Natl. Acad. Sci. USA, 85:8790-8794 (1988); Agrawal, S. et al., J. Cell Biology, 107:468 (1988); Haralambidis, J. et al., Nucleic Acids Res., 18(3):501-505 (1989)), intercalating (Helene, C. and J. J. Toulme, Oligodeoxynucleotides-Antisense Inhibitors of Gene Expression, Ed. J. S. Cohen, Macmillan Press, 137-166 (1989)) and chelating (Oser, A. et al., G. Nucleic Acid Research, 16:1181-1196 (1988)) reagents attached to synthetic oligonucleotides are becoming important tools of molecular biology. A variety of enzymatic and chemical procedures have been developed for their synthesis (Matthews, J. S. and L. J. Kricka, Anal. Biochem., 169:1-25 (1988)). Central to some of these procedures are (a) the introduction of a reactive group at either the 3'- or 5'- terminus of the oligonucleotide (Agrawal. S. et al., Nucleic Acid Research, 14:6227-6245 (1986); Agrawal, S. Tet. Lett., 30:7025-7028 (1989); Fidanza, J. A. and L. W. McLaughlin, J. Am. Chem. Soc., 111:9117-9119 (1989); Nelson, P. S. et al., Nucleic Acid Research, 17:7187-7194 (1989)) or (b) the synthesis of modified nucleosides which contain the masked reactive group and are incorporated into the nucleic acid (Fidanza, J. A. and L. W. McLaughlin, J. Am. Chem. Soc. 111:9117-9119 (1989)). The presently-available methods are useful, but are limited in their usefulness for site specific internal non-radioactive labelling of synthetic oligonucleotides possible.